The present disclosure relates generally to devices and methods for relieving disc degeneration or injury, and more particularly, to devices and methods for augmenting or replacing a nucleus pulposus. Within the spine, the intervertebral disc functions to stabilize and distribute forces between vertebral bodies. The intervertebral disc has a nucleus pulposus which is surrounded and confined by the annulus fibrosis (or annulus).
Intervertebral discs are prone to injury and degeneration. For example, herniated discs typically occur when normal wear, or exceptional strain, causes a disc to rupture. Degenerative disc disease typically results from the normal aging process, in which the tissue gradually loses its natural water and elasticity, preventing the degenerated disc from maintaining the normal separation of vertebral bodies.
The interior portions of intervertebral discs of the spine are not provided with a significant blood supply by the body. Their homeostasis is aided by the diffusion of fluids into the disc tissue, thus supplying them with nutrients. This, to some extent, allows the tissue to grow and repair damage done by stress as the intervertebral joint moves. Despite this process, in mature adults, spinal disc tissue degrades continuously over time. Degenerative disc disease typically results from the normal aging process, in which the tissue gradually loses its natural water and elasticity, causing the degenerated disc to shrink. Sufficiently advanced degeneration can lead to herniation or rupture of the spinal disc. In addition to normal wear and tear, herniated discs can occur from exceptional strain or trauma.
Herniation of a spinal disc can result in a number of debilitating symptoms, including intractable pain, weakness, and sensory loss. Treatment of these symptoms frequently requires surgical removal of at least a portion of the herniated disc, a procedure known as discectomy. Often, discectomy alone cannot stop the progressive degeneration at the level of disc excision. An additional procedure often is performed in conjunction with the discectomy with the objective of fusing together the vertebral bodies surrounding the affected disc space. This is accomplished by removing the cartilaginous portion of the endplates by scraping the surfaces of the vertebral body and inserting a piece of graft bone, which may be an allograft from a bone bank, or an autograft, typically taken from the iliac crest of the patient, or other suitable material.
Fusion procedures, however, can be problematic. Even when successful, the grafting or fusion procedure requires considerable recovery time before fusion is complete. Perhaps even greater concern, fusion procedures eliminate normal spinal biomechanics. Range of motion at the level of the fusion typically is eliminated because the affected vertebrae have been effectively joined to form a single bone structure. Because the patient tries to maintain the same overall range of motion of the entire spine, additional stress is imposed on the intervertebral discs of the adjacent vertebrae. This, in turn, may lead to accelerated degeneration at levels above and below the fusion site, which may require additional treatment, including discectomy and fusion at those adjacent levels. Additionally, grafting procedures carry some risk of tissue rejection and disease transmission if an allograft is used, and risk of harvest site morbidity when the patient's own tissue is used.
As a result of these difficulties with intervertebral fusion, attempts have been made to provide a prosthetic solution to degenerative disc disease that maintains the patient's normal spinal biomechanics, allows for shorter recovery times, and avoids the complications inherent in harvesting and/or grafting bone tissue.
Design and construction of such an implant, however, is not simple. Desirably, the implant should be precisely placed in a prepared intervertebral space, and should contain elements that are immobilized with respect to each of the vertebral bodies, so that the implant does not migrate or shift, potentially contacting, abrading, or otherwise damaging the spinal cord, ligaments, blood vessels, and other soft tissue. At the same time, the implant should allow the vertebral bodies to move relative to each other in a way that provides the equivalent motion afforded by a healthy intervertebral disc, and that allows the affected vertebral joint to participate in the coordinated overall movement of the spine in a way that closely approximates the natural movement of a healthy spinal column. The implant should be biocompatible, and avoid the introduction of toxic or harmful components into the patient, such as release of wear debris. The implant also should restore normal disc height and maintain the patient's vertebral lordosis, and should not allow any significant post-operative subsidence (protrusion or movement of the disc into the adjacent vertebral bodies) or expulsion (protrusion or movement of the disc outside of the disc space). The implant should be at least partially constrained by soft tissue in and around the intervertebral space, in order to allow a simpler, more efficient design. Further, such an implant also should ideally provide elasticity and dampening sufficient to absorb shocks and stresses imposed on it in a manner similar to that of the natural spinal disc.
In addition to the above requirements, there remains a need for a device which would decrease patient recovery time, and reduce the occurrence of postoperative degeneration at levels above and below the implant, as compared with fusion devices and techniques as well as existing motion-preserving devices. There also is a need for a device that does not require any significant preparation of the disc space prior to implantation. That is, some existing devices require significant shaping of the vertebral endplates prior to implantation for proper placement of the device. It is, therefore, desirable to provide a device that maintains motion and requires relatively little or no shaping of the endplates prior to implantation. It is desirable, however, for the same device to maintain proper positioning inside the disc space, restoring normal disc height and maintaining the patient's vertebral lordosis, lessen the chances for post-operative subsidence or expulsion, create relatively little or no wear debris, and provide elasticity and dampening sufficient to absorb shocks and stresses imposed on it in a manner similar to that of the natural spinal disc. Satisfying these requirements will provide a very high quality disc that also is easy to implant.